The MRI apparatus is an apparatus which measures an NMR signal generated by the subject, especially, the nuclear spins which form human tissue and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding and different frequency encoding are given to NMR signals by the gradient magnetic field. Measured NMR signals are reconstructed as an image by two-dimensional or three-dimensional Fourier transform.
The measured NMR signals (hereinafter, referred to as echo signals) are digitized and arrayed as echo data in a k space (data space). In this case, if echo data with a change in the shape of the subject caused by breathing movement is arrayed in the same k space, motion artifacts appear on an image reconstructed from such k space data and lower the diagnostic performance. In order to reduce such motion artifacts, a respiratory gating method, a diaphragm navigator method, and a breath-holding method are used.
The respiratory gating method is a method of connecting a respiratory sensor to the subject to measure a breathing cycle and measuring the echo data only in a specific phase. For this reason, the echo data of the different respiratory phases is not mixed in the k space. As a result, motion artifacts can be reduced.
The diaphragm navigator method is a method of measuring the diaphragm position of the subject by measuring the echo data called navigator echoes and of measuring a breathing cycle from the position change. Although the method of measuring a navigator echo is the same as the respiratory gating method, it has a feature that a respiratory sensor is not necessary.
Finally, the breath-holding method is a method of making the subject stop breathing and measuring the echo data in the meantime. Thus, the breath-holding method is the simplest method because a respiratory sensor or acquisition of additional data is not required, and its clinical use is also widespread. Since one measurement needs to be performed within a time for which breath holding is possible in measurement using the breath-holding method, the breath-holding time is shortened by devising a sequence or a reconstruction method (parallel imaging or the like) in order to shorten the measurement time (Non-patent Documents 1 and 2).
Through combination of these known techniques, Patent Document 1 discloses a technique of repeatedly instructing the subject to perform breath holding and free breathing and performing imaging when body movement detected by a navigator echo after a breath-holding instruction falls within a predetermined range.